Antenna system



Aug. 28, 1945. I J. MADl DOCK 3 ANTENNA SYSTEM Filed March 10, 1945 Attorney Patented Aug. 28, 194

AN'IEIYHJ'A SYSTEM Alan Julian Maddock, London O. 2, England assignor to Standard Telephones and Cables Limited, London, England; aBritish company Application March 10, 1943; Serial No. 478,683

In Great Britain March 27 1942,:

5 Claims. ('01. so fr)" In radio antenna systems which have directive properties it is often desirable and in many cases necessary that the antenna system should emit radiation of one characteristic polarisation only whilst radiation of any otherpolarisation should 1 be suppressed, e. g. in a system employing ver. tical antennae it is desirable that only waves of vertical polarisation should be emitted whilst all horizontal polarisation be suppressed and vice versa.

Polarisation of opposite characteristic can arise from currents induced in objects such as screens,

-towers,-etc. which may be present and which can radiate in the unwanted mode. As an example, a directive antenna system consisting of vertical elementsjplaced above the horizontal screened roof of a building or vehicle will radiate mainly waves of vertical polarisation but due partly to the limitation in size of such roof or screen and partly to the different potentials assumed by the various antennae elements near to said surface, currents will flow in the surface and may give rise to radiation which is' horizontally polarised. Similarly an array of horizontal elements supported by structures such as metal poles or towers i will radiate principally waves of horizontal polarisation but currents will be set up in the vertical members which may then radiate in the vertical mode. A single vertical antenna if located with one end in proximity to a conductor or sheet of limited dimensions can also give rise to horizontal polarisation due to the concentration of the current in the sheet morein one direction than in others. 1

In many applications it is necessary to suppress these unwanted radiations and a particular example of a system in which it is essential that tennasystem, the roof of said building orv'ehicleoften'beingscreened or covered .with ,mesh, for reasons we1l.known to thoseexperiencedin the 7 art. As explained above currents set upLinv the horizontal roof .orscreen williradiate horizontally polarised waves and itisusually'the casethat thepolar patterns of'the radiation emittedz in this sense are not-thesame as those of the'primary'emission, in this casethat due to the vertical antennae. Itis apparent then that. should the receiving antenna not be vertical, for example in the case of an aeroplane when it banks, then a component of horizontally polarised; radiation I will be received and it may be-tha't the-signal will be of different character, e'.-:g.'-dots may be received due tothe horizontal polarisation where dashes would be receivedldue to the vertically polarised radiation. .Similar remarks apply when the main radiation is horizontally polarised. lt'is' the object of this invention to provide atmeans for eliminating" the radiation of unwanted characteristic at least over a considerable solid angle where its elfects would be most harmful and'in most instances it is found that immunity can be obtained; at all points if conditions are satisfied at the most crucial points.

According to the; present invention, in an .an-

- 'tenna system comprising one or more antennae this should be done is that in which two or more vertical elements are used to radiate electromagnetic waves which have intersecting polar diagrams in a horizontal plane and inwhich radiation is modulated in some rhythmic sequence between the two patterns, for example inthe form of dots and dashes, so that an aircraft, fitted with suitable receiving apparatus-which includes a vertical receiving antenna-receives dots or dashes according to which side of the intersecting line it is located andalo'ng the line of intersection and for a small angle each side of which the dots and clashes merge into a continuous signal so giving an approach line for the aircraft along which it can be guided. Such systems are well known and in the past have usually taken the form of half-wave vertical elements supported at least a half wavelength above the earth and connected to the transmitter b suitwhich has or-have' one end thereof in close proximity to a conducting sheet or other conducting system from which radiation is-emitted polarised in a difierentsense from'the radiation emitted by the main antenna system, one' 'or'a plurality,

of conducting screens'o'r sheetsl'orplurality of conductors are so disposedrelative tothe first mentioned conducting sheet I or system that shielding of the radiation from said firstrmen 'tioned conducting sheet or system is obtained within a desired solid angle. V I

'The invention will be better understood from thefollowing description; taken in conjunction with the accompanying drawing in'which Figure 1 shows a dipole antenna system located'above a and X' Y which are symmet icalabout' the points O and O and may'for example be half-waveantennae The method of feeding power to the an;

tennae is not shown as it'is immaterial to-th'e'invention and may be carried'out in well-known ways such as centrefeeding the antennaeat'the points 0 andO', or end feeding at the points Y and Y. or feeding one and exciting the other parasitically and so on. The antennae are considered located above the surface or conductor MN which, as stated hereinbefore may be the roof of a building, vehicle or part of the antenna supporting -minate on the conductor MN a difierence of potential will exist between the points M and N and a current I will flow in this conductor as shown by the arrow.

Fig. 2 is a view end-on to the antenna system of Fig. 1 and the current in the sheet MN flows perpendicularly to the paper at N. This current gives rise to radiation pricipally in direction at right angles to itself, horizontally polarised. If the points in space in which we are mainly intere'sted in the radiation from the vertical antennae, be perpendicular to the plane of the antennae then-it is possible to eliminate the unwanted radiation at least within a certain solid angle; this 'may be sufficient for certain .purposes where, for example, we are only concerned with receiving the vertically polarised radiation within certain limits of angle. For example, if an aeroplane is being guided by means of the radiation from the vertical antennae along a line perpendicular to the plane containing them,-only radiation within an angle RNP is effective since even with an altitude of 10,000 feet at 10 miles distance, this angle is only 11. Thus if we can eliminate the horizontal polarisation within this angle the system will be satisfactory.

The invention consists in placing a conducting sheet or'screen PQ of such a size and in such a position relative to the position of the sheet or conductor MN t'h'at screening of the surface in which the unwanted'current is flowing takes place over a certain angle PNQ in the vertical direction and the angle XSX' in the horizontal direction (Fig. 3'). Thus within .the solid angle bounded by the lines subtended by the limits ofthe sheet or screen PQ at the horizonta1 axis of the unwanted current and the axisof the vertical antennae, horizontally polarised radiation will be materially or completely reduced. Improvement in the reduction of the'unwanted radiation can often be effected by the use of a plurality of sheets or screens and as an example a second screen is shown at P Q. By this means currents induced in the screen MN can be made, to have a much smaller effect within the said solid angle since radiation set up by such currents is reflected back by the screen or screens PQ. I

It is also possible to reduce the radiation in the opposite direction NR' by adjustment of the spacing NR such that the currents set up in the screen PQ by the current in MN in conjunction with the distance NR can be made to be in opposite phase and thus cancel, at least in part, as far as radiation is concerned in this general direction. It is not usually possible to effect a complete elimination of reduction in direction NR in this way since the current in PQ will generally be smaller in magnitude than that in MN so that although phase reversal can be effected complete cancellation will be absent due to inequality of the currents. A greater current can however be induced in the screen PQ by making the length such that it is resonant or nearly so at the operating frequency when more exact cancellation can be obtained.

In the case considered of the half-wave anten however, where the conductor MN forms part of the counterpoise for the radiating elements and i where the current loop occurs at the base of the antennae some loss of field strength may occur but cancellation of the horizontal polarisation can again be effected in the manner already indicated.

It will be observed that the invention is equally applicable to horizontal antennae when correction must be applied to eliminate vertical polarisation. In this case KY and X Yare horizontal radiating elements and PQ- and MN vertical members. Furthermore, extension to inclined antennae will also be understood to those skilled in the art.

Whilst the foregoing description deals with half-wave and quarterwave antennae the invention is equally applicable to antennae of any length.

The invention may also be applied to cases in which correction is required for all directions about the central axis of the antenna system. Then screening sheets of conductors ma be placed all round the conductor MN so that shielding is effective in all or any desired direction.

What is claimed is:

1. An antenna system comprising an antenna having a polarized radiation characteristic and a conducting surface arranged in close proximity to one end of said antenna and having a radiation characteristic polarized along an axis of polarization different from that of the radiation characteristic of said antenna, and conducting means extendingv across a predetermined solid angle formed around said axis of polarization for shielding said antenna from the radiation polarized along said axis of polarization.

2. An antenna system as claimed in claim 1, in which the conducting means is a screen intersecting the conducting surface.

3. An antenna system as claimed in claim 1, in which the conducting means comprises two conducting screens intersecting the conducting surface and placed one behind the other.

4. An antenna system as claimed in claim 1, in which the conducting means comprises a screen having a radiation characteristic polarized along an axis substantially parallel to that of the radiation characteristic of said antenna and intersecting the conducting surface at a distance such that currents set up in said screen by said antenna and by the current in said conducting surface are substantially opposite in phase.

5. An antenna system as claimed in claim 1, in which the conducting means comprises a screen substantially resonant at the operating frequency of the antenna and having a radiation char acteristic polarized along an axis substantially parallel to that of the radiation characteristic of said antenna, said screen baing arranged intersecting the conducting surface at a distance from the antenna such that currents set up in said screen by said antenna and by the current in said conducting surface are substantially opposite in phase, to reduce radiation in a direction opposite to said solid angle.

ALAN JULIAN MADDOCK. 

